Feedback control loops, and particularly phase-locked loops, often benefit from the utilization of variable bandwidth filtration to secure the combination of rapid initial control acquisition and subsequent tracking noise rejection.
For instance, in the case of information storage unit drives which use magnetic disc type storage media, it is common to include a phase-locked feedback control loop which comprises a phase-locked variable frequency oscillator (VFO) system which is locked in phase to a signal retrieved from the storage disc in the drive. The phase-locked loop (PLL) typically generates a control signal proportional to phase error between a signal which is read from the disc, a PLL reference signal, and a signal generated by a voltage-controlled oscillator (VCO). The PLL control signal is used to shift the frequency of the VCO to reduce the detected phase error difference to an acceptably low level. The output of the VCO is used to supply clocking that allows data detection. Filters are included in this feedback control system to tailor the response time of the VCO output signal to changes on the input.
It is generally desirable to provide a wide bandwidth for the PLL control signal to provide a rapid acquisition for the PLL control system. The wide bandwidths and high PLL control system gain necessary for this purpose are achievable with good system design. However, such high bandwidths and gain cause the VCO to vary in phase and frequency in proportion to PLL control signal noise which is generated by noise in the PLL reference signal. This noise can then cause corresponding extraneous high frequency phase shift of the VCO signal, which can cause undesirable effects.
Therefore, once the VCO is locked to the PLL reference frequency after the beginning of the preamble field is read, it is desirable to reduce the bandwidth of the PLL control signal for the remainder of the preamble field to eliminate this high frequency noise. Generally, the additional bandpass filtering necessary for this purpose is minimal since the preamble field contains only the single frequency PLL control signal. However, the data field includes a wide range of signal frequency components which introduce large peak phase shift noise on the PLL reference signal, which causes amplitude noise on the PLL control signal. Furthermore, since the VCO is already locked onto the PLL reference signal by the time that the data field is read from the disc, only long term variations in phase shift need be corrected. Consequently, relatively narrow bandpass filtering is desirable while the data field is read to discriminate against noise, since the limited bandwidth then has no adverse effect upon maintaining PLL control.
The multiple bandpass filter systems according to the prior art generally can provide such response characteristics, but they all have significant disadvantages. They typically use a master filter with characteristics to define the maximum PLL control signal bandwidth during acquisition, with one or more auxiliary filters to successively couple onto the master filter for progressively reducing the PLL control signal bandwidth. The switching process between the master filter and the auxiliary filters causes serious discontinuities in the output of the filter system. This is because energy storage devices in the auxiliary filter systems must be charged to provide output levels matching the final value of the master filter before switching. Discontinuities cause transients in the output of the VCO. Such discontinuities can be minimized by initializing the energy storage components. Circuits for initializing the energy storage devices, which are typically larger than the master filter, are complex, and active devices for rapidly charging the energy storage devices, such as high speed operational amplifiers, are very expensive, or are of high power and unsuitable for integrated circuits. The discontinuities can also be minimized by just reducing the gain of the system, which causes a reduction in system bandwidth, but this also causes the "damping" of the system to be reduced. Insufficient system damping allows excessive overshoot response.